1. Field of the Invention
The present invention relates to dynamic light scattering particle size distribution analyzers.
2. Description of the Related Art
The so-called “dynamic light scattering particle size distribution analyzer”, as disclosed in Japanese Patent Laid-Open Publication No. 2002-221479, is known as a device for measuring the particle diameters of small particles. This type of device analyzes a particle size distribution based on the following measuring principle. That is, in a sample containing a certain concentration of particles to be measured, the particles move irregularly (Brownian motion) in a solvent with the tendency that relatively large particles move slowly while relatively small particles move quickly. When the sample is irradiated with laser light having a fixed frequency, the laser light is scattered with the particles. Since the particles are individually in Brownian motion as described above, the frequency of scattering light from each particle shifts slightly from the frequency of incident light depending on the motion velocity of each particle due to Doppler shift.
Therefore, measurement of such shifts in frequency makes it possible to determine the particle size distribution of the sample statistically. Specifically, the total sum of frequency shifts of scattering light from all the particles is observed as a “fluctuation” in the intensity of scattering light and, hence, the particle size distribution of the sample can be calculated from such a fluctuation by using an algorithm exclusive to this type of calculation.
However, such scattering light takes place not only at the particles to be measured but also at various other sites such as scratches of a cell containing the aforementioned sample, the interface between the cell and outside air or between the cell and the sample, and the like. Such scratches and interfaces are fluctuated due to 1/f fluctuation occurring in nature, minute fluctuations of the cell, presence of surface tension wave, or the like. Scattering light from such scratches and interfaces are also subject to frequency shifts due to such fluctuations and hence cause noise to occur, which affects the measurement.
With a sample having a relatively high concentration, scattering light from particles to be measured are predominant. Signals generated by noise light scattered from the aforementioned scratches and the like are of negligible levels because they are buried in optical fluctuation signals from the particles. However, in the case of a sample having a relatively low concentration of particles to be measured or a sample containing fine particles, signals generated by noise-causing scattering light is of levels that cannot be neglected and as a whole suffers from such a problem as to lower the measurement precision or the SN ratio, which might result in impossibility of effective measurement.
With a view to overcoming this problem, a dynamic light scattering particle size distribution analyzer has heretofore been developed of the immersed cell type in which a cell is immersed in a liquid having a refractive index approximately equal to that of the cell in order to reduce noise-causing scattering light occurring at the interface between the outside surface of the cell and outside air due to a difference in refractive index.
Such an analyzer, however, cannot effectively eliminate noise-causing scattering light caused by scratch or deflection of the cell wall and noise-causing scattering light occurring at the interface between the inside surface of the cell and the sample contained therein, though it is capable of eliminating noise-causing scattering light occurring at the interface between the outside surface of the cell and the outside air interface. Particularly recently, heavy use has been made of a disposable cell made of a resin such as a plastic. Such a cell is susceptible to scratch, deflection, or streak resulting from heterogeneity of a plastic and appearing like a string in the cell wall, which are responsible for the occurrence of noise-causing scattering light
Further, the immersed cell type analyzer often uses an organic solvent to immerse the cell. Such an organic solvent is difficult to handle and often requires that the structure of the analyzer be made complicate. In addition, dust or contaminant, which is possibly mixed into the solvent, may cause noise-causing scattering light to occur.
Accordingly, it is an object of the present invention to provide a dynamic light scattering particle size distribution analyzer having a very simple structure capable of reducing noise-causing scattering light that is responsible for the occurrence of noise.